Astronauts’ bone density doesn’t bounce back

A year back on Earth is still not enough to recover from a few months of bone density loss in space, according to new research on 17 astronauts.

Living in zero gravity has a number of effects on the body, including a loss of bone density. A study published in Scientific Reports has found that this density usually isn’t completely restored, 12 months after a 4-month (or more) sojourn in space.

“We’ve seen astronauts who had trouble walking due to weakness and lack of balance after returning from spaceflight, to others who cheerfully rode their bike on Johnson Space Center campus to meet us for a study visit,” says co-author Dr Steven Boyd, a professor in the Cumming School of Medicine at the University of Calgary (UCalgary), Canada. “There is quite a variety of response among astronauts when they return to Earth.”

“Bone loss happens in humans—as we age, get injured, or any scenario where we can’t move the body, we lose bone,” explains lead author Dr Leigh Gabel, an assistant professor in the Faculty of Kinesiology, also at UCalgary.

“Understanding what happens to astronauts and how they recover is incredibly rare. It lets us look at the processes happening in the body in such a short time frame. We would have to follow someone for decades on Earth to see the same amount of bone loss.”

The researchers scanned the tibia (shin) and radius (forearm) of 17 astronauts (three female, 14 male) before they went to space. They then did the same scans three times on their return: straight after getting back, then six and 12 months after. The scans were able to tell them about the bones’ mineral density, resistance to fractures, and tissue thickness.

“We found that weight-bearing bones only partially recovered in most astronauts one year after spaceflight,” says Gabel.

“This suggests the permanent bone loss due to spaceflight is about the same as a decade worth of age-related bone loss on Earth.”

All of the astronauts’ missions lasted between four and seven months. The eight astronauts who stayed in space for longer than six months had notably less bone recovery.

In better news for the spacefarers, some exercises during spaceflight seemed to mitigate this bone density loss and improve recovery a little. Astronauts who stepped up their in-flight resistance training – deadlifting – were better at recovering bone mineral density in their shins.

More on astronaut bone density: Space lettuce that stimulates bone growth

The findings also confirm that astronauts respond differently to the same amount of time in space, with some showing much better recovery than others.

“Just as the body must adapt to spaceflight at the start of a mission, it must also readapt back to Earth’s gravity field at the end,” says Dr Robert Thirsk, a former UCalgary chancellor and former astronaut. “Fatigue, light-headedness, and imbalance were immediate challenges for me on my return.

“Bones and muscles take the longest to recover following spaceflight. But within a day of landing, I felt comfortable again as an Earthling.”

The researchers are next hoping to examine astronauts who’ve been on even longer space missions.

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